Luminescent tube for wave indication



Oct 22, 1935. o. EMERSLEBEN ,3

LUMINESCENT TUBE FOR WAVE INDICATION Filed March 51, 1932 2 Sheets-Sheet 1 INVENTOR OTTO EMERSLE BEN ATTORNEY Oct. 22, 1935 0. EMERSLEBEN 5 2,017,897 LUMINESCENT TUBE FOR WAVE INDICATION I Filed March 51', 1932 2 Sheets-Sheet 2 o o 44 o O O INVENTOR OTTO EMERSLEBEN ATTORNEY Patented Oct. 22, 1935 UNITED STATES PATENT OFFICE wn'nnsscsm: runs roa wava mnrca'rron I Otto Emersleben, Berlin-Zehlendorl, Germany, assignor to Radio Corporation oi America,- a corporation of Delaware Application March 31, 1932, Serial No. 002,113 In Germany April 13, 1931 6 Claims.

The present invention relates to frequency indicators utilizing tubes wherein gases underreduced pressure are caused to become luminous. It is known in the prior art, for instance, in Lecher wire systems, that radio frequency electromagnetic oscillations may be indicated by means of luminous actions merely by bringing the two current supply leads (electrodes) of a Geissler tube or a similar gas-filled tube or vesactual contact with a part carrying the oscillations. This mode of indication was effective in cases where the electromagnetic field was strong enough to cause luminescence of the gas enclosed in thevessel also outside the conducting parts. However, whenever the electromagnetic fields are weak and'not sufiiciently'intense, the luminous actions are not capable of being produced, while an arrangement carrying radio frequency energy is generally unsuited to cause excitation of luminescence with two-electrode luminous tubes. In fact, such an arrangement may essentially be used only when working with waves of a length of an order of magnitude comparable with the dimensions of the apparatus, so that there are caused to exist pairs of points on the apparatus which have opposite potentials whereby there are produced short circuits through pairs of electrodes with the connected luminous tubes.

In other words, an arrangement of the sort known in the prior art was useful only for oscillations which, compared with the apparatus,

may not be regarded any longer as quasi-stationary However, all technically or commercially important radio frequency oscillations in- -.volve quasi-stationary working conditions so that the use of this known method is precluded alone for this reason.

' It has also been suggested in the prior art (luminous quartz crystal method ofGiebe and Scheibe) to have recourse to piez'o-electric crystals for the detection of electrical oscillations of a definite frequency which operates as a resonator in a rarefied rare gas atmosphere, the

rare gas being caused to luminesce at points nodes. It has also been found (by Eberhard) that in order to produce luminous actions it is not necessary, when using a quartz resonator,

\ to directly introduce the crystal into the rare gas atmosphere, indeed, that under definite presup- 5 positions and conditions it is sufilcient that the quartz crystal should be disposed outside the vessel in the immediate vicinity of the glass wall of the vessel filled with the rare gas atmosphere. ,In fact, with such an arrangement it is even 10 ieasible to arrange one of the electrodes by which the quartz is caused to oscillate, with the oscillations in turn producing luminescence of the gas. To be sure, this scheme requires that the crystal be ground to a certain thickness in 15 order to be responsive to a definite wave length, but it was otherwise not adaptable enough to permit of its further'uses. At any rate, in this arrangement only those parts of the rare gas at? mosphere were caused to luminesce which were 20 located in the direct neighborhood of the ,quartz, to be more precise, directly at the glass wall, and this means an impairment of the chances for observing luminous actions of the type here 25 dealt with.

Now, the present invention obviates the disadvantages of such a known arrangement, and is adapted to indicate and detect radio frequency oscillations of relatively low energy or power by rendering these oscillations perceptible by luminous actions. 4

According to the invention, a conductive part of the circuit arrangement which carries radio frequency oscillations is in single pole connection with a closed transparent'container or vessel con-.

taining vapors or gases adapted to be rendered luminescent by radio frequency oscillations; Such single pole connection may consist of a short wire or lead, the latter, however, is made so short in length that an adequately large ampli- 40 tude is introduced into the gas filled vessel. The latter may consist, for instance, of a glow discharge (gaseous-conduction) lamp or tube equipped with a pair of helical or loop-shaped electrodes, the vessel being filled with neon or a 45 neon-helium mixture. 0! the two current supply leads of a standard glow discharge lamp the outer one is connected by means of a short and relatively stout copper wire of a few centimeters length to a point of the circuit arrangement which 50 is a potential nodal point for the radio frequency oscillations. Suited for this purpose is a point in the circuit arrangement which is not grounded and which in reference to the ground presents as high as possible a resistance; for example, in the 55 denser connected with the grid of the tube. In

contrast to arrangements known in the earlier art (Lecher wires) where for the operation of the scheme the existence of two points of opposite potential changing in their potential at a high rate of frequency was the prerequisite condition, actual experiments have shown that with the use of suitable luminous tubes a luminous action is producible even when the rare gas vessel connected with a single point of the circuits presenting marked amplitude fluctua ions of the radio frequency energy. At first blush this'appears surprising in that the determination of the electric potential save for one constant is arbitrary, and as it is wholly feasible to make his constant dependent upon time. Hence, it is possible to regard the ground as a constant potential and to cons der the radio frequency oscillations of a conductor part in relation to the said ground potential; but it would be just as wellto regard these conductor parts being insulated from the ground as be ng at cons ant poten'ial, and to speak comparat vely of rad o frequency oscillations of the grounded parts. Although this latter mode of looking at the situation is not conventional or customary it would nevertheless be justified from a physical viewpoint. Hence, it would at first not be plausible for what reasons, in the case of single pole connection of the luminous vessel with the circu ts. a luminous phenomenon should arise just when the parts insulated against earth are united with the tube in one pole. It is this feature that distinguishes the present idea from a two-pole circuit scheme known in the prior art, for instance. in the use of Lecher wires in which, independently of the selection of the datum or reference point for making the potential meas urements, there exists a potential fluctuation between the two electrodes occurring at high frequency. However, experimentation shows that,

quite surprisingly, under conditions as before' stated, also a single pole connection sufiices for obtaining luminosity.

An a posteriori explanation the inventor believes to be as follows: While the luminous tube is only in single pole connection with the circuit arrangement, to be more precise, with the part thereof which is insulated from ground, the other parts of the luminous tube, 1. e., the electrode positioned opposite the other pole, other metallic parts, etc., however, are inductively acted upon by the ground potential for the reason that in the inductive actions the larger mass of the surrounding (ambient) parts being at earth or other stationary potential predominates compared with the relatively feeble inductive actions from the conductor parts insulated from the ground and possibly in single pole connection with the glow discharge vessel. Hence, even if one were to assume the viewpoint that the potential of such insulated parts is to be regarded as constant, and correspondingly the earth potential, the potential of the wires, etc., as varying at a high frequency, I

then also those parts of the glow discharge tube which are in single pole connection with the insulated circuit point would have to'be regarded as being at a constant potential. However, owing pro to the said inductive actions from the grounded parts the opposite metal parts or other metal parts present inside the glow discharge tube and not in conductive relationship with the enclosed electrode would undergo inductive radio frequency potential variations.- In other words, no matter how the situation be viewed, 1. e., for every choice of the zero potential (including one su ject to time variation) radio frequency electromagnetic oscillations between the conductor parts of the luminous tube must be expected in an arrangement according to the invention involv- 5 ing single pole connection, with the result that the gas is luminesced as the experiment has evidenced.

Among the points of the circuit arrangement insulated from ground where asingle pole tube 0 connection should most' preferably be effected there are to be recommended particularly such points of the circuits where, by the aid of special auxiliary .means, high potential amplitudes are obtainable. What may be particularly recom mended for this object are piezo-electric crystals exisent in thewave generation scheme for the purpose' of frequency stabilization. Compared with luminous resonators known in the prior art, the present invention, quite apart 20 from the constructional arrangement of the quartz (which in the present scheme is not so restricted as to its arrangement as is true of luminous resonators) distinguishes itself inter alia owing to the fact that the inventionis use- 25 ful also in connection with oscillators. Whereas in resonators it is only certain and definite points of the quartz that give rise to luminosity in the neighboring gases, while other points of the quartz will not participate in this phenome- 30 non, any desired point of an adequately insulated electrode of the quartz may be united with the luminous vessel or tube in orderto cause the latter to luminesce, while the other electrode will not manifest a luminous action or will luminesce 35 only to a far slighter degree even if it is insulated though not to the same extent.

The idea underlying this invention is schematically illustrated in the accompanying drawings wherein Figures 1 and 2 are shown by way 40 of example. Figs. 3 and 4 illustrate other .embodiments utilizing the present invention.

Figure 1 shows a wave generating circuit arrangement, wherein it is proposed to indicate the generated waves by a luminous action. Re- 45 ferring to Figure 1, i denotes a triode transmitter comprising a filament 2, a grid 3 and a plate 4. The heating of the filament 2 is effected from a battery 5; the plate circuit contains a plate potential source 6, and between the latter and 50 the plate 4 is provided a tuned oscillation circuit comprising a coil 1 and a rotary condenser 8. Between the grid 3 and the filament 2 is disposed a quartz crystal oscillator comprising a crystal 9, the coat in on the grid end and the 5 coat ii on the'filament end. The plate'potential source is shunted or bridged by a condenser I 2. The grid coat iii of the crystal 9 is united by a short and stout copper wire i 3 with one pole iii of aneon glow-discharge lamp ll, where- 60 as the other pole is left unconnected. If the other pole of the gaseous-conduction lamp is associated with the filament end coat ll of the quartz crystal oscillator, there arise also luminous actions, to be sure; but by the glow dis- 55 charge lamp there is set up a resistance, though of high ohma'ge, between the coats i0 and H of the quartz crystal oscillator adapted to diminish' the ensuing amplitudes.

Figure 2 shows the arrangement of the quartz and the luminescent tube drawn to a somewhat larger scale in section. To be sure, it is possible to unite the crystal coat directly with the luminous tube by way of a short wire; but it is sufficient if of the two plug pins l9 and i6 whereby 7 the crystal oscillator is secured in the circuit, the one on the grid end indicated at I!) is united with the metal wire ill, or if the terminal pin I! brought to the grid of the transmitter valve is connected with the luminous tube by way of a short metallic wire. A conductive sheath or envelope laid around the glass vessel such as an external tin foil coat l8 tends to enhance the luminous phenomena inasmuch as it. increases the inductive actions from the outside. For the same reason it is recommendable to dispose the inner electrode-lead near the glass walls in direct proximity to the conductive sheath. I The invention has been tried out in the presence of circuit elements having, e. g., the follow-'- ing dimensions. What was used was a quadratic quartz plate 9 possessing a natural period of 100 m (thickness about 0.9 mm). .The oscillation circuit 1, 8 consisted of a coil comprising 10 spires and a frequency-proportional rotary condenser of a maximum value of 500 cm. The shunting condenser I! was of l mfd. Instead of an oscil ator tube or transmitted valve it was sufficient to use a simple receiver tube RE604 or else RE134 (manufactured by Telefunkeni.

The same was operated with a 4--V accurnulatorplate current as a result rises very markedly,

being 42 and 23 milliamps, respectively, for the two tubes, whereas in the resonance setting of the oscillatory state is amounted to only 5.3 and 1.5 milliamps, this minimum of the plate current value arising in the presence of a condenser position of 61 and 64 degrees, respectively. Uponfurther reduction of the condenser value 8, the plate current exhibits another rise in accordance with the departure of the natural period of the oscillation circuit 1, 8, from the'natural frequency of the quartz crystal 9.

The luminous tube, when using the same copper connection wire of '3 cur length and the tube RE134 lights up only when the condenser is in the resonance position at about 60 degrees.

When employing tubeRE604, the glow discharge tube luminesces also at all lower positions of the condenser. Hence, the invention makes it feasible to indicate resonance both by the arising of a potential crest and exact resonance as wellas in the presence of a'region characterized by increased potenital variations, independently of the particular point involved. By lengthening the copper wire brought to the glow discharge lainp it is possible to damp the radio frequency lations to a point so that also when using'tube RE604 luminescence ofthe gas will happen only when the condenser is set inthe resonance position.

purposes of establishing the said connectiom' Hence. according to the invention it is possible to influence and control the damping of the radio frequencylead by choosing corresponding dimensions for the length of the Supply lead. Inas- .much as it is possible'to choose a length as great as 30 cm. which is considerable for radio frequency circuit schemes it is possible in practicing to the constant carrying of the headset of tele- This condition ariseswhen the connecting wire the invention to use and connect the glow discharge lamp at any desired point in the scheme without any particular limitations being imposed.

In contrast with the luminous resonator types disclosed in the earlier art which operate only in 5 the direct: neighborhood of the piezo-electric crystal, the arrangement of this invention allows of a 'great deal of freedom as regards the disposition of the lamp or: tube. For instance, in

practicing the invention there is no inconvenience 10 in mounting one of the glow discharge tubes out-.- side the apparatus, the latter containing the assembly of the circuits, to thus indicate the oscillatory state of the The arrangement is useful not only for self-excited transmitters, 15 but also for separately excited transmitters as well as receiver sets, and it may also be employed for the purpose of functioning as a signalling means or the like to indicate the presence of oscillations of a definite frequency. For instance, 20

a luminous device of the kind here disclosed may be used for controlling a telephone or a loudspeaker to show whether oscillations are present L at all in case the telephone or loudspeaker fails to indicate oscillations. It is also possible in case of non-use of the loudspeaker or headset to cut this lamp in circuit in order to save current (the consumption by the glow discharge lamp being extremely small) or else to prevent fatiguing due 30 graphic operators and the like. By the use of the invention it is also possible to dispose upon a common switch panel for different transmitters and receivers and other apparatus carrying radiofrequency oscillations, controllingmeans, supervisor lamps, etc. for the various apparatus provided 1 that, in the presence of a plurality of apparatus, the necessary shortening of wires is taken into consideration. The glow discharge lamp thus excited, because of its low consumption, may also 0 be used directly for illuminating purposes.

One embodiment of such a check-upor supervising equipment is schematically shown in Figures 3 and 4. Figure 3 shows an arrangement comprising. two transmitters which are to be checked up as regards their state of oscillation by a comprehensive lamp device. Figure 4 is a view of a suitable switch panel. 1

Referring to Figure 3, suppose S1 and S: are two transmitters which'by the aid of a corresponding go quartz crystal Q1 and Q1, respectively, are to be maintained at a certain transmitter frequency. In each of the transmitters S1 and $1 the grid end coat of the crystal Q1 and Q2, respectively, is in conductive relationship with two glow dis- 55 charge or gaseous-conduction lamps G11 and G1:

'for the transmitter 81, and Gm and-G21 for the transmitter S1; Of these the pairor glow discharge tubes, the ones first named, i. e., glow discharge lamps G11 and Gui have a short connection so lead to the coupling electrode of the crystal; and the other one pair, 1. e., the glow discharge lamp G1: and G112, a longer lead. The envelopes of these lamps consisting of a conductive coat is indicated in Figure 3 by two parallel-lines, and 05 in the elevation Figure 4 by a double circle. By

, choosing diflerent dimensionsfor the supply leads,

say, for the lamps G11 and G21 for, the transmitter .81. it is possible to insure the following conditions:

Each length of the connecting wire corresponds to a definite threshold value of the oscillation amplitude; and it is only when the amplitude of the osclllations atthe connected quartz crystal coat exceed this threshold value that the glow discharge lamp will flash up. By choosing different threshold values which is easily accomplishable by a corresponding alteration of the length of the lead or connecting wire it is possible to insure several advantages, it having to be noted in this connection that it is possible to use more than two different glow discharge lamps, that is to say, more than two threshold values for one transmitter for the purpose of indication, etc.

A piezo-electric crystal, as is well known, serves for the stabilization of the wave generated by I an oscillator insofar as the frequency of this wave,

under the influence of the crystal, becomes equal to the natural period or one of the natural periods of the quartz crystal whenever the natural period of an oscillation circuit provided in the transmitter exercisinga frequency-governing effect'exhibits certain deviations from the natural period of the crystal. An oscillation circuit in the transmitter consisting, for instance, of a condenser and inductance coils, under the action of quite a number of external agencies, such as the temperature, atmospheric pressure, mechanical percussions of the apparatus, and so on, is very apt to experience changes in its natural period. It is well known also that an alteration of resistance in the oscillation circuit may occasion not only damping, but under certain circumstances also a change in the natural period of this particular circuit. In all of such cases where, under the influence of uncontrollable and inadvertent changes of the oscillation circuit quantities. the natural period of the circuit undergoes alterations, or if under the influence of intentional or anticipated changesin damping (caused, e. g., indirectly by modulationor the like) there occurs thus a disturbing change in the natural period, recourse may be had, as is well known in the art, to the use of a wave stabilizing piezo-electric crystal, which in its own motion is relatively little impeded by outside agencies, and which in addition may be provided extensively with special ways and means in order that such slight influences as may exist may be precluded, for instance,

byaccommodating the crystal inside an ambient independent of the temperature, thermostats or melting ice or inside a pressure-independent ambient or confinement in vacuo or the like. To be sure, these auxiliaries insure that within certain limits, i. e., as long as the departures of the frequency of the oscillation circuit from the natural period of the crystal keep inside a certain easily insurable limit, the frequency of the wave will be constant and equal to the natural period of the quartz crystal.

But the amplitude of this oscillation nevertheless will be dependent upon the difierence between the natural frequency of the quartz crystal and the natural period of the oscillation circuit or circuits co-determinative in-the frequency of the oscillation. 1

Hence, the situation may arise that, on relying upon the frequency-stabilizing action of the oscillator crystal such fluctuations as are ascribable to or producible by temperature fluctuations or the like in the natural periods of the oscillation circuit or circuits are wholly neglected or at least not fully appreciated, with the result'that in the course of a gradual temperature change the amplitude of the oscillation is subjected to considerafie variations and liable to fall appreciably below .the desired or attainable value.

Now, this troublesome occurrence may be counter-acted, for instance, by an arrangement as shown in Figures 3 and 4.- With this end in view it may be advisable to choose the lead of one oi the glow discharge tubes, e. g., G11 to the crystal so short inlength that relatively low amplitudes 5 vof the oscillations will occasion luminescence of the glow discharge tube. Hence, this particular glow discharge tube will make it possible to tell whether oscillations are present at all. Hence,

the glow discharge tube G11, when arranged-as 10 shown in Figures 1 and 2, would be'caused to luminesce provided that the setting of the rotary condenser 8 lies somewhere between 63 and 67 degrees. It is only when this limit has been ex ceeded, i. e., when the oscillations are broken of! 15 entirely and when the considerably larger emission current corresponding to the static characteristic flows through the tube, that the glow discharge tube G11 would cease luminescing. Of course, the said tube or lamp will fail to lu- 20 minesce at all when the generator is not in circuit. The second glow discharge tube G12 on the other hand has a connecting wire brought to the crystal Q1 of such a length that only in the neighborhood of the real resonance position will this 25 glow discharge tube be caused to luminesce, i. e., when the amplitude of the generated wave is equal to the desired maximum or at least in its direct neighborhood, in other words, in the example illustrated in Figure 1 and Figure 2 in the30 presence of a condenser position of 61 and 64 degrees, respectively. If on the panel Figure 4 no glow discharge tube is caused to flash up at all this goes toindicate that the transmitter does not oscillate at all or 35 at least not at appreciable amplitude. If solely lamp G11 luminesces, but not tube G12, this must be interpreted to mean that the transmitter is oscillating, but with an unduly weak amplitude. This may mean that disconnection should'be ef- 40 fected. A difierential photometer or the like may be so set that it will be caused to respond when one of the glow discharge lamps, i. e., lamp G11 is caused to luminesce, and not the other one, G1z,'whereby a signal device in the operators 45 room is actuated whence the reason for the failure of the desired amplitude being produced is ascertained. The reason, inter alia, may be that one of the tuning means has experienced an unintended mechanical shift. In most instances, U0 however, the setting of the tuning elements may have remained constant, though the value thereof may have been affected or changed by external agencies such as temperature fluctuations and the like. On the panel Figure 4 is thus shown below 6 the glow discharge lamps G12 and G1: a Vernier setting means A1 and A2, respectively, say, for a condenser by the aid of which readjustment is ac complishable, so that the respective glow discharge lamp G1: or G112, as the case may be, is 6 caused to reluminesce. On the panel Figure 4 there is furthermore shown below the glow discharge lamps G11 and G111 a switch E1 and E2, respectively, which is used for connecting the entire transmitter assembly. .-In the place 65 thereof there could also be provided an indicator device such as a glow lamp or thelike to show whether the transmitter is in circuit or not. When the same is connected, but when the glow discharge lamp G11 or G21 indicates the absence 70 of waves, this means that the transmitter is out of order, for instance, on account of the fact that the natural period of the oscillation circuit has so much changed in one direction that the oscilla- Figure-1 this would happen upon a change in the sense of increased capacity or reduced natural frequency of the circuit.

The arrangement as hereinbefore disclosed is not confined to the special embodiments here described'in connection with the transmitter. In fact, the invention may be used, e. g., with receivers, resonator crystals, etc. in schemes which may be superior to the luminous crystals resonators. In the case of the luminous resonator, the presence of the luminous action is the criterion for very sharp frequency coincidence. The result is that whenever there occurs a comparative-- ly slight frequency change, the crystal will fail to luminesce-any more. Hence, it is not feasible to ascertain from the quartz crystal itself whether oscillations exist at, all; and after this has been ascertained by some other ways and means, whether the wave length is too great or toosmall. In previous practice one remedy of the situation has'been to use a plurality of luminous crystals, say, three or five, of which the middle one had a natural frequency the same as the desired frequency, whereas the other crystals had a natural period slightly difiering therefrom,

' for instance, with equal distances between the natural waves of these luminous crystals. How- 'ever, even in such a scheme it has been found quite oftenthat no luminescence was produced, and there was no other way out of the difiiculty than to reset, detune, or the like, in order to get into one of those very small or narrow resonance ranges inside which one of the luminous crystals would flash up. Hence, it was often enough only possible after a makeshift detuning to determine whether oscillations existed at all, and then ascertain by observations as to which one of the various crystals would luminesce to thus determine the sense 'of the departure or detuning.

These difliculties are obviated also by the present invention. In fact, by the use of the invention, and the provision of a correspondingly long supply lead or wire and a suitable metallic sheath or electrode shape it is possible to choose the range inside which the glow discharge lamp, in the presence of oscillations of the crystal or other circuit point carrying wave amplitudes, be caused to luminesce. But if also smaller amplitudes acting at the crystal coat are to be ascertained, all that is necessary with that end in view is to choose the wire to the glow discharge tube V correspondingly shorter.

1. An arrangement for indicating radio frequency oscillations of low power comprising a gas filled glow tube having a transparent, hermetically sealed envelope containing gas therein and which is adapted to exhibit luminous phenomena, a wire within said envelope, a conducting band adjacent said envelope and insulatingly positioned with respect to said wire, a source of radio frequency oscillations of low power, and an electrical connection from saidv source to said wire, said band being physically separated from and not connected to said source, the length of said connection determining the threshold value of the amplitude 'of the oscillation which it is desired to detect.

2. An arrangement 'for indicating high frequency oscillations comprising a gas filled, transparent, hermetically sealed envelope adaptedto exhibit luminous phenomena, a wire within said envelope connected to a terminal outside said envelope, an external band of tinfoil surrounding 5 said envelope, adjacent thereto, and insulatingly positioned with'respect to said wire, an oscillation generator comprising an electron discharge device having a cathode, a control electrode, and a piezo-"electric crystal located between said control electrode and the cathode, and a connection between said wire and that side of said piezoelectric crystal which is connectedto the control electrode of said electron discharge device, the length of said connection determining the threshold value of the amplitude of the oscillations which it is desired to indicate.

'3. An arrangement for indicating radio frequency oscillations of low power comprising a plurality of gas filled glow tubes each of which comprises a hermetically sealed envelope having a wire therein and extending externally thereof, and a conducting band adjacent each of said envelopes and insulatingly positioned with respect to itsassociated wire, a piezo-electric crystal resenator having a plurality of electrodes, and individual connections of predetermined lengths from one of said electrodes'to the wires of said glow tubes, the striking of the glow in said tubes being determined by the lengthsof said individual connections, said conducting bands being separated from and not connected to any of the circuit elements of saidarrangement. 4. An arrangement for indicating radio frequency oscillations of low power comprising a glow tube having a hermetically sealed envelope, a wire within said envelope extending externally thereof, a circular conducting band adjacent said envelope and insulatingly positioned with respect to said wire, a piezo-electric crystal having a plu- 40 rality of electrodes and a connection from one of said electrodes to said wire, said connection being of predetermined length and the only one from said glow tube to the circuit elements of said arrangement, the length of said connection deducting-band adjacent the envelope of said tube and insulatingly positioned with respect to said wire, a secondsimil'arly constructed glow tube,

a source of high frequency oscillations, and indi-. vidual electrical connections of predetermined diilerent lengths from saidsource to the wires of said respective tubes, the conducting bands of said tubes being physically separated from and not connected to said source, the lengths ofsaid connections being such that said first glow tube indicates theproduction of oscillations in said source, and said second'fglow tubeindicates the attainment of a predetermined amplitude of the oscillations in said source. 

